When quantum dots meet blue phase liquid crystal elastomers: visualized full-color and mechanically-switchable circularly polarized luminescence

Polymer-based circularly polarized luminescence (CPL) materials with the advantage of diversified structure, easy fabrication, high thermal stability, and tunable properties have garnered considerable attention. However, adequate and precise tuning over CPL in polymer-based materials remains challenging due to the difficulty in regulating chiral structures. Herein, visualized full-color CPL is achieved by doping red, green, and blue quantum dots (QDs) into reconfigurable blue phase liquid crystal elastomers (BPLCEs). In contrast to the CPL signal observed in cholesteric liquid crystal elastomers (CLCEs), the chiral 3D cubic superstructure of BPLCEs induces an opposite CPL signal. Notably, this effect is entirely independent of photonic bandgaps (PBGs) and results in a high glum value, even without matching between PBGs and the emission bands of QDs. Meanwhile, the lattice structure of the BPLCEs can be reversibly switched via mechanical stretching force, inducing on-off switching of the CPL signals, and these variations can be further fixed using dynamic disulfide bonds in the BPLCEs. Moreover, the smart polymer-based CPL systems using the BPLCEs for anti-counterfeiting and information encryption have been demonstrated, suggesting the great potential of the BPLCEs-based CPL active materials.


S3
Synthesis of disulfanediylbis(ethane-2,1-diyl) diacrylate (DSDA): The synthetic route of crosslinker DSDA is shown in Scheme S1.Bis(2-hydroxyethyl) disulfide (250 mg, 1.62 mmol), TEA (330 mg, 3.24 mmol), and dry THF (4.0 mL) were added into a 25 mL Schlenk flask.Acryloyl chloride (590 mg, 6.40 mmol) was dropwisely added to the solution under a nitrogen atmosphere.The flask was sealed, and the reaction mixture was stirred at 0 °C for 24 h.The mixture was allowed to be warmed to room temperature.The resulting precipitated salt was removed through filtration, and the filtrate was washed with an aqueous sodium carbonate solution (0.1 M).The organic layer was dried over MgSO4 and concentrated on a rotary evaporator.

Calculation of activation energies (Ea)
Stress relaxation experiments were performed on the DHR-2 rheometer with a constant strain (γ=1%) at varying temperatures.As shown in Figure 4c, the relaxation time τ* of QD-BPLCE films was determined at 110 °C to 130 °C.The relaxation time is plotted against 1000/T and fits the Arrhenius relationship in Equation S1.
* () =  0    / (Equation S1) Where  0 is the relaxation time at infinite T, Ea is the activation energy of the transesterification (kJ mol -1 ), R is the universal gas constant (8.314J K -1 mol -1 ), and T is the temperature (K).Equation S1 can be transformed to Equation S2.

Welding performance of QD-BPLCE
In the current system, the content of disulfide bonds is relatively low, and to ensure the ordered structure of QD-BPLCE, the processing temperature cannot exceed the phase transition temperature (140 °C), which is lower than the traditional disulfide bond S14 processing temperature (200 °C).Therefore, the welding of QD-BPLCE must be carried out under the condition of an external catalyst. [1]1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU) was the catalyst that accelerated the disulfide bond exchange rate.DBU (10 mg, 0.46 mmol) was dissolved in 10 mL dimethyl sulfoxide as the "glue."A few microliters of the above catalyst were lightly brushed onto the interface between overlapping QD-BPLCE using a capillary.The film was welded at 110 °C for one hour to achieve film welding.Figure S20 (Supporting Information) shows the SEM image of the crosssection of the overlapping part before and after film welding.Before welding, there were significant gaps between the films, indicating that the two films were independent.
After welding, the two layers were connected into a complete unit, meaning the two films were successfully welded together.Figure S21 (Supporting Information) shows the mechanical properties of QD-BPLCE before and after welding.Compared with the unwelded film, the elongation at the break of the welded film they decreased to 80%.
The mechanical performance weakening may be caused by QD-BPLCE being welded in a parallel overlapping way, which also needs to consider the influence of shear modulus in the stretching process.References: (1) Chen, L. et al.A cut-and-paste strategy towards liquid crystal elastomers with complex shape morphing.Journal of Materials Chemistry C 2018, 6, 8251-8257.

1.
Photoinitiator Irgacure 651 was purchased from BASF Company.The chemical structures of the materials for the fabrication of the QD-BPLCE film are shown in Figure S1.

Figure S1 .
Figure S1.Chemical structures of the components for the fabrication of the QD-BPLCE film.

Figure S6 .
Figure S6.POM images of G-G-BPLCE during the heating process at different temperatures.

Figure S10 .
Figure S10.The glum of G-G-BPLCE by changing the angle of the sample along the direction of incident light propagation.

Figure S15 .Figure S16 .
Figure S15.The cyclic performance of stretch-induced disappearance of CPL signals.

Figure S19 .Figure S20 .
Figure S19.Photographs and SEM images of the cross-section (a) before welding and (b) after welding.